Perhaps the first step in discussing this subject is to first get the terminology correct. The latest revision of Lycoming Service Instruction No. 1324 calls it a crankshaft oil seal. Although almost everyone knows exactly what you are talking about when the term nose seal is used, correct terminology can be important. Should this seal leak immediately after installation, it is possible that the seal was damaged during the installation process, but a poor fit between the crankcase and seal or the crankshaft and seal could also be responsible for the leak.
Before installation of a crankshaft oil seal, it is important to check the recess into which it fits for proper size. Excessive wear which enlarges the crankcase bore for any reason may cause the crankshaft oil seal to leak. An undersize crankshaft could result in the same poor fit and a leak. This is usually caused by a rusty or pitted surface which has been polished excessively. It also prescribes the procedures for replating the crankshaft flange and seal area. Measurement of both the crankshaft and the crankcase to ensure that they meet Table of Limit tolerances should be standard maintenance procedure before installation of a crankshaft oil seal.
To avoid damage during installation, it is important to follow the instructions provided in the latest revision of Lycoming Service Instruction No. 1324. This instruction gives information on the two types of seals which may be used in Lycoming direct-drive engines, the part numbers of both the standard and the oversized seals, and the method of installing both types of seal.
When a leak at the crankshaft oil seal develops after many hours of normal operation, it is usually the result of other problems. The experienced Field Service Engineers at Lycoming indicate that a leaking crankshaft oil seal is frequently caused by a restricted breather or an oil-slinger clearance that is too tight. The leak might also be caused by a propeller defect which places an abnormal side load on the crankshaft oil seal.
To avoid the problem of oil leakage at the crankshaft oil seal because of an engine breather restriction, examination of the breather tube to determine its condition is an excellent idea. If the tube is in good condition, also remember that the engine expels moisture through the tube. Under freezing conditions, there is some possibility that the moisture may freeze at the end of the tube and ice will build up until the tube is completely restricted. Should this happen, pressure may build up in the crankcase until something gives – usually the Crankshaft Oil Seal.
Since the airframe manufacturers know this is a possibility, and since they design with the intention of preventing engine-related problems of this kind, some means of preventing freeze-up of the crankcase breather is usually a part of the aircraft design. The breather tube may be insulated, it may be designed so the end is located in a hot area, it may be equipped with an electric heater, or it may incorporate a hole, notch or slot which is often called a “whistle slot.” Because of its simplicity, the whistle slot is often used and is located in a warm area near the engine where it will not freeze. Aircraft operators should know which method of preventing freeze-up is used and then ensure that the configuration is maintained as specified by the airframe manufacturer.
Should leakage at the crankshaft oil seal occur as a result of oil-slinger clearance which is too tight, the problem can initially be identified by checking for excessive end clearance. This can be done with a dial indicator. Remove the prop and then push the prop flange to the extreme aft position and zero the indicator. Then, pull the prop flange full forward and read the travel on the indicator. Compare this figure with the limits listed in the Table of Limits for the appropriate engine model.
Should the end clearance exceed the limits specified, the oil-slinger clearance should then be checked. First, remove the old nose seal, and clean the work area. Again, push the crankshaft to the rear of the engine. Insert a .002 inch feeler gauge about 3/16-inch wide between the oil slinger on the crankshaft and the crankcase. Again, pull the crankshaft forward. If the .002-inch feeler gauge is pinched tight, the required .002- to .007-inch clearance has been exceeded. Lack of appropriate clearance is the result of excessive wear on the crankcase thrust face which will allow oil to be pumped out past the crankcase oil seal.
Overhaul time is usually when the crankcase thrust face might receive needed repair. Should the crankcase oil seal be leaking excessively, it may simply mean that overhaul time has arrived early. Fortunately this is something which does not happen very often.
Crankcases with worn or damaged thrust face areas can be repaired by reworking the thrust face area to permit installation of new thrustbearing washers. These bearings are available as repair items. Thrustbearing washers may be reused if they do not show wear and if their thickness is sufficient to maintain compliance with the crankshaft and crankcase end-clearance specifications in the Lycoming Table of Limits.
Instructions for repairing the crankcase thrust face are found in the latest revision of Lycoming Service Instruction No. 1545. This repair should bring the oil-slinger clearance back into tolerance and should cure the leaking crankcase oil seal which would also have been replaced during the reassembly of the engine.
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Service Instruction No. 1324
For more information on engine leaks, please refer to the related service publications.